There are known processes to prepare arylphosphines by reacting, for example, phosphorus trichloride with a Grignard reagent or with an organolithium compound, followed by hydrolysis, extraction and distillation. These processes have disadvantages, in that materials used in the processes are expensive, corrosive, difficult to prepare owing particularly to their sensitivity to moisture, and cumbersome to handle on a large scale.
In one aspect the present invention provides a process for preparing a mono or di-arylphosphine, which process comprises reacting an aryl compound bearing a leaving group attached to a carbon atom of the aromatic ring with phosphine in the presence of a Group VIII metal catalyst.
In accordance with the invention it is possible to avoid or alleviate many of the abovementioned disadvantages by reacting phosphine with an aryl compound in the presence of, preferably, a palladium catalyst.
A particular advantage of the present invention is that it is possible to obtain mono-arylphosphines and di-arylphosphines, i.e., primary and secondary arylphosphines, in good yield, and accompanied by only relatively small amounts of triaryl, i.e., tertiary phosphine. Owing to the high reactivity of phosphorus compounds, reactions to form arylphosphines usually proceed quickly to the triaryl compound. Consequently, triarylphosphines are available much more readily and cheaply than the corresponding mono- and di-arylphosphines. The mono- and di-arylphosphines are of considerable interest, particularly as intermediates and also as components of catalyst for many reactions. Their possibilities have not been exploited, however, owing to their relative inaccessibility and high price. By means of the present invention, and by careful selection of reaction conditions such as temperature and pressure, it is possible to influence the relative amounts of mono-, di- and tri-arylphosphines produced.
The aryl compound can have only carbon atoms in the ring, or can be heterocyclic containing one or more nitrogen, oxygen or sulphur atoms. As nitrogen-containing compounds there are mentioned, e.g. pyridine, pyrimidine, piperazine, pyrazole. As an oxygen-containing heterocyclic compound there is mentioned furan. As a sulphur-containing heterocyclic compound there is mentioned thiophene. Examples of hydrocarbyl aryl compounds include phenyl, xcex1-naphthyl, xcex2-naphthyl, biphenyl, phenanthrenyl, anthracenyl, naphthacenyl and 2,2xe2x80x2-bis(1,1xe2x80x2-binaphthyl) groups.
Preferred leaving groups are the halogens, particularly chlorine, bromine and iodine. Other suitable leaving groups include, for example, trifluoromethane-sulfonyloxy, methanesulfonyloxy, toluenesulfonyloxy and trifluoroacetate groups. The leaving group is attached to a carbon atom of the aryl ring. The aryl compound can bear one or more than one leaving group. Examples of aryl groups that bear two leaving groups, and therefore may bear two phosphorus atoms after reaction, include the 1,2-phenyl group, the 1,4-phenyl group, the 2,2xe2x80x2-biphenyl group of formula 
and the 2,2xe2x80x2-bis(1,1xe2x80x2-binaphthyl) group of formula 
The aryl halide is preferably an iodo- or a bromo-compound. The aryl moiety can be unsubstituted or can be substituted by groups that do not interfere with the reaction. Such substituents include hydrocarbyl groups such as alkyl, alkenyl and cycloalkyl groups. Mention is made of alkyl and alkenyl groups, straight chained or branched, having up to about 8 carbon atoms, cycloalkyl groups having from 3 to 8 carbon atoms and aryl groups such as phenyl or naphthyl, aralkyl groups such as benzyl or phenethyl and alkaryl groups such as tolyl or xylyl groups. Other substituents include acyl, acyloxy, alkoxy, alkenoxy and aryloxy groups, again having up to about 8 carbon atoms. Particular compounds include bromotoluenes, bromoxylenes, iodotoluenes and iodoxylenes. The preferred aryl halides are bromobenzene and, especially, iodobenzene.
As stated above, the aryl compound can bear substituents that do not participate in the reaction with phosphine. It is found that the reaction of the present invention goes better with electron-withdrawing groups, for instance trifluoromethyl, cyano, alkylcarbonyl and alkoxycarbonyl. The substituted compounds that are of greatest interest, however, are those that bear electron-donating groups, for instance lower alkyl and lower alkoxy groups. The aryl compound can bear one, two or more substituents. To avoid steric interference it is preferred that the substituents shall be in the 3-, 4- or 5-position, relative to the leaving group. Mention is made of 3-trifluoromethyl-phenyl, 4-trifluoromethylphenyl, 3-cyanophenyl, 4-cyanophenyl, 3-acetylphenyl, 4-acetylphenyl, 3-methoxycarbonylphenyl, 4-methoxycarbonylphenyl, 3-acetoxyphenyl, 4-acetoxyphenyl, 3-methylphenyl, 4-methylphenyl, 3,5-dimethylphenyl, 3-methoxyphenyl, 4-methoxyphenyl and 3,5-dimethoxyphenyl groups, and also aryl groups other than phenyl that are correspondingly substituted.
Phosphine is a gas under ambient conditions. The reaction with the aryl compound is preferably carried out under elevated temperature and pressure. The product of the reaction is a mixture of the mono-, di- and tri-aryl phosphines and the relative ratio of these three products varies, depending upon the temperature and pressure of the reaction, the amount of catalyst and the presence of a base as promoter. The particular reaction conditions selected will therefore depend upon the desired ratio of these products. Elevated temperature, decreased pressure, a greater amount of catalyst and a greater amount of base promoter all tend to increase the amount of the di-aryl product at the expense of the mono-. Temperatures from 70xc2x0 C. to about 150xc2x0 C. are suitable, preferably from about 90 to 120xc2x0 C. The pressure may be in the range from ambient to about 600 psig.
The reaction is carried out in the presence of a catalyst that is a metal of Group VIII of the Periodic Table, or a compound of such a metal. Most suitable are the precious metals of Group VIII, of which palladium is preferred. Mention is made particularly of zero valence compounds of palladium examples of which include tetrakis(triphenyl-phosphine)palladium, 1,2 bis(diphenylphosphine)ethane palladium, dichlorobis(triphenylphosphine)palladium, 1,3-bis(diphenylphosphine)-propane palladium, 1,4-bis(diphenyl-phosphine)butane palladium and 1,1-bis(diphenylphosphine)-ferrocene palladium.
Mention is made particularly of adducts of a Pd(II) salt and a tertiary phosphine, especially 1:1 adducts as catalysts. As palladium salts there are mentioned the diacetate and the dichloride. The tertiary phosphine can be a trialkylphosphine, for instance, tri(ethyl)-, tri(propyl)-, tri(n-butyl)-, tri(isobutyl)-, tri(cyclopentyl)-, tri(cyclolhexyl)- and tri(n-octyl)phosphine or a triarylphosphine, of which triphenylphosphine and tri(ortho-tolyl)phosphine are preferred. One preferred catalyst is an adduct of palladium (II) acetate and tris(o-tolyl)phosphine and is described (in German) by Wolfgang A. Herrmann et al., in Angew. Chem., 1995, Volume 107, pages 1989-1992 and (in English translation) in Angew. Chem. Int. Ed. Engl., 1995, Volume 34, pages 1844-1848, the disclosure of which is incorporated by reference.
Mention is made also of palladium compounds that are standard xcfx80 donors, for instance Pd[cyclooctadiene]2, Pd[cyclopentadiene]2 and Pd[dibenzylacetone]2.
The amount of catalyst can range from about 0.05 mole to about 10.0 mole percent, preferably from about 0.1 to about 7.5 mole percent of the aryl compound initially charged.
It is possible to form the catalyst and then add it to the reaction vessel, or it is possible to add the components of the catalyst, so that the catalyst is formed in situ.
The reaction is preferably carried out in the presence of a solvent. Suitable solvents include glyme, acetonitrile, diethyl ether, anisole, di-n-butyl ether, tetrahydrofuran, p-dioxane, toluene, xylene, cumene or a mixture of toluene and isopropanol (e.g. a 3:1 mixture). Also suitable are aliphatic, cycloaliphatic and aromatic hydrocarbons, including hexane, heptane, octane, cyclohexane, benzene and petroleum fractions boiling at 70-140xc2x0 C. Solvents that have oxidising properties, such as DMSO, should be avoided. Toluene is most preferred.
As stated above the reaction is preferably carried out in the presence of a base promoter such as for example sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, calcium carbonate, sodium bicarbonate, potassium carbonate, sodium ethoxide, potassium ethoxide, ammonium carbonate, ammonium bicarbonate, calcium oxide, calcium hydroxide, magnesium oxide, magnesium hydroxide or the like. Organic bases, particularly amines, can also be used. Mention is made of pyridine and pyridine derivatives and tertiary amines of which triethylamine, tributylamine, and other trialkylamines are preferred.
The phosphine is supplied to a closed reactor under pressure that is suitably in the range of from about 100 psig to 600 psig, preferably about 200 psig to about 500 psig. The temperature of the reactor is elevated to at least about 80xc2x0 C. but does not usually exceed about 150xc2x0 C. Preferably the temperature is in the range of from about 1000 to about 150xc2x0 C. The reaction time may be up to about 6 hours, after which the reaction is quenched. Quenching is suitably carried out by cooling. The reactions between the aryl compound and phosphine do not proceed at reasonable rates much below about 90xc2x0 C., so cooling the reaction mixture to below about 70 or 80xc2x0 C. is effective. Shorter reaction time and less base both favour the production of mono-arylphosphine over di- and tri-arylphosphine, so these parameters can be varied in accordance with the desired product mixture.
The invention is further illustrated in the following examples.